Experimental Investigation of The Effect of Bed’s Coarse Gravel on the Deposition and Entrapment of Suspended Cohesive Sediment Process

Bed material is one of the factors affecting the deposition and entrapping of cohesive sediments. In this work, entrapment of cohesive sediment in gravel bed (D50= 2.4 and 9.4 mm) and smooth bed was investigated using circular flume. For this purpose, the effect of gravel bed sediments on the entrapping and deposition of suspended sediments in different concentrations and different flow turbulence has been tested. The experiments were carried out in two modes of smooth and rough beds for initial concentrations of 5, 10 and 20 g/l and flow velocity 0.14, 0.24 and 0.34 m/s. The results showed that, in two modes of bed, the concentration of suspended sediments decreases with time, but the bed contains coarse sediment increases the amount of deposition rate and the time of reaching to an equilibrium concentration. The results also showed that the entrapping coefficient in bed containing sand and gravel beds was about 5.7 and 3.5 times more than that of smooth bed, respectively. Increasing the entrapping coefficient in larger shear stresses can be explained by the effect of coarse-grained bed sediments on reducing the flow velocity and generating bursting - sweep phenomena due to flow turbulence.